In particular, the present invention concerns a method for cool drying gas, in particular air, which contains water vapour, whereby this gas is led through the secondary part of a heat exchanger, whose primary part is the evaporator of a cooling circuit which also comprises a compressor driven by a motor; a condenser; an expansion means between the outlet of the condenser and the inlet of the above-mentioned evaporator.
Such methods, that are known among others from BE 1,011,932, are used among others for drying compressed air.
Compressed air, supplied for example by a compressor, is in most cases saturated with water vapour, or in other words has a relative humidity of 100%. This means that when the temperature drops under what is called the dew point, there will be condensation. The condensed water causes corrosion in pipes and tools, as a result of which the appliances may wear prematurely.
That is why compressed air is dried, which may be done by means of the above-mentioned cool drying. Also other air than compressed air or other gasses can be dried in this way.
Cool drying is based on the principle that, by lowering the air or gas temperature in the evaporator, moisture in the air or gas will condense, after which the condensed water is separated in a liquid separator and after which the air or the gas is heated again, as a result of which the air or gas will be no longer saturated.
The same is also true for other gasses than air, and each time we refer to air hereafter, the same also goes for any other gas than air.
A method for cool drying is already known, whereby based on measurements of the evaporator pressure or the evaporator temperature, the cooling circuit is switched on or off.
If it is found that there is a decrease of compressed air, the cooling circuit will be started and as soon as the take-off of compressed air stops again, also the cooling circuit is stopped again.
A disadvantage of such a known method is that the heat exchanger, after the cooling circuit has been switched off, will heat up as cooling is no longer available.
If, subsequently, compressed air is taken off while the heat exchanger is still relatively warm, temperature and dew point peaks may immediately occur in the supplied compressed air, since the gas to be dried in the heat exchanger will not be sufficiently cooled then to make the water in the gas to be dried condense at maximum capacity.
In Belgian patent application No. 2005/0310 is described a method for cool drying that offers major improvements compared to the conventional methods.
To this end, the method of BE 2005/0310 consists in measuring the temperature of the dew point in the environment of the place where the temperature of the gas to be dried is the lowest when cool drying, and in switching on and off the cooling circuit so as to always maintain the lowest gas temperature of the dew point between a predetermined minimum and maximum threshold value, whereby these threshold values are calculated on the basis of an algorithm that is a function of the measured ambient temperature.
By the lowest gas temperature or LAT is meant here the lowest temperature of the gas to be dried that occurs while cool drying and that is reached, in principle, at the outlet for the gas to be dried of the secondary part of the heat exchanger. The LAT always gives a good indication of the dew point of the gas, since there is a connection between both.